This invention relates generally to conduction mode control in switching converters.
A switching power converter is a multi-port network having at least two ports, at least one of which is an input and at least one of which is an output. Inputs absorb electrical power from an external source; outputs deliver electrical power to an external load. The converter is typically a network of reactive elements, switching elements and, in addition, and sometimes one or more transformers. The reactive elements include at least one inductor, and possibly one or more capacitors. The switching elements include at least one externally-controlled switch. The externally-controlled switches are driven by a control circuit which adjusts the duty cycle of the switches and possibly the timing relationships between various switches so as to regulate the flow of electrical power through the converter. The converter will assume each of its topological configurations as determined by the control circuit. The mode of operation of the converter may also be continuous or discontinuous. In a continuous mode of operation, the inductor always carries an electric current. Conversely, in the case of a discontinuous mode of operation, the switch is activated again only after the discharge current of the inductance has become null.
One typical converter topology is a buck converter. Traditional buck topology DC to DC converters, shown in FIG. 1a, utilize a “freewheeling” diode (35) as a return path for when the energizing switch (26) is open. This is also true for other topologies including forward, flyback and boost. When the load current is high enough the inductor current always flows from the power supply to the load. In this case, either the source switch (26) is on or the inductor current forces the freewheeling diode (35) on. At light loads, the inductor is discharged to a point that it reaches zero current. At this time the freewheeling diode is reversed biased and no longer conducts. This can be seen in FIG. 1. There are advantages and disadvantages to these two modes of operation. The primary advantage is that at light load the inductor current is lower and thus the efficiency is higher. The primary disadvantage is that current cannot be removed from the load and results in overcharging the output capacitor. The only current removal mechanism is the load itself. This results in a slow transient response.
To maintain high efficiency and high load currents and/or low output voltages, buck regulators, as shown in FIG. 2, commonly use mosfet switch (28) instead of a diode. The low side (LS) mosfet switch is termed a synchronous rectifier. One method to control the synchronous rectifier is to drive it with the complement of the source side control with the provision being that there is a non overlap time to control cross conduction or shoot-through. The primary advantage of this approach is much higher efficiency at high loads and faster trends in response because the output capacitor can be actively discharged.
It would be desirable to have Synchronous rectifier operation at high loads during continuous inductor current conduction and diode like operation with light loads during discontinuous inductor conduction. There are conventional designs that attempt this mode of operation.
One conventional method ensures that the inductor current is always positive, in the direction of the load; hence the supply is always in continuous conduction. In this method, the inductor current is modeled by a circuit, either analog or digital, and the time at which the discharging inductor current reaches zero is predicted as in FIG. 3. At the predicted time, the synchronous rectifier is turned off. Because the voltage across the inductor is dominated by the change in the switch node, the derivation of the inductor current can assume that the output voltage is constant during one PWM cycle. This assumption must be valid for a useful power supply with a small output voltage ripple.
In another conventional method, the current in the synchronous rectifier is measured by monitoring the voltage drop a cross the synchronous rectifier during the time when it is on. When the current reaches zero the synchronous rectifier is turned off until the next pulse width modulation, PWM, cycle. One disadvantage of this method, is that for high efficiency power supplies the voltage across the synchronous rectifier is very small. This is particularly true for systems that use low on resistance external mosfets. In that case, the voltage drop across the synchronous rectifier caused by the inductor ripple current may only be a few millivolts. This small signal is very difficult to measure with a low delay. The comparator required for this task is costly and requires significant power. In addition, the comparator power source commonly has significant noise due to the fast switching speeds and high power levels. This noise can easily trip the negative current comparator and cause erroneous operation.